1. Technical Field
The present invention relates to a manufacturing control system and a manufacturing control method for growing a single crystal in which the resistivity is controlled to a desired value.
This application claims priority from Japanese Patent Application No. 2006-011908, filed on Jan. 20, 2006, the content of which is incorporated herein by reference.
2. Background Art
In recent years, there has been a desire to increase the capacity of LSI devices, and the development of manufacturing technology for achieving this end is also required. Large numbers of silicon wafers are used in manufacturing technology for semiconductor circuits such as IC and LSI devices.
Such silicon wafers are fabricated by adding a given proportion of impurity to a silicon starting material and creating by crystal growth a cylindrical crystal body called a single crystal which has a desired resistivity, cutting the single crystal into a cylindrical body of a given length in the axial direction called an ingot, then slicing the ingot into sections of a required thickness.
One method for growing such single crystals is the Czochralski method (referred to below as the “CZ method”). The grown single crystal is sliced to form wafers having the qualities required by the user (e.g., thickness, diameter dimensions, resistivity, and etch pit density), and then the wafers are shipped to the respective users.
However, as shown in FIG. 8, in a single crystal that has been manufactured, there are regions where the qualities required by the user are not met; that is, portions where the electrical characteristics are not fully satisfied, such as quality loss (resistivity) and EPD loss (the etch pit density exceeds a prescribed value). Or there are portions which, although satisfactory in terms of these electrical characteristics, are not monocrystalline on visual examination, and portions where dislocations have occurred or cracks associated with such dislocations exist. Moreover, there are portions of the single crystal where the diameter falls outside of the error range and does not satisfy the prescribed values, namely the conical top portion where crystal pulling and growth was begun (top loss) and the conical bottom portion where crystal growth was brought to an end (bottom loss).
The above-mentioned quality loss, EPD loss, top loss and tail loss portions which do not meet the quality requirements of the user become, of course, unshipped material which is not sliced into wafers and shipped as finished product.
In order to grow a single crystal for such wafer production and also achieve a predetermined impurity concentration, use is made of high-purity impurity materials and silicon materials. In particular, impurity materials are expensive, as are also silicon polycrystalline materials because of their high purity. Hence, discarding such unshipped material amounts to throwing away high-cost impurity material, which increases the production costs involved in growing a single crystal, and in turn drives up the production costs for wafers.
In order to lower the proportion of the production costs accounted for by the impurity material, cyclic production methods that reuse the impurity material present within the above-described unshipped material are being adopted.
In general, the portions of an ingot which satisfy the quality requirements of the user are sliced from the ingot to prepare wafers, leaving behind the above-described unshipped material portions as reusable ingots. Such reusable ingots held in stock are used during recycling (e.g., see Patent Document 1).
In the method of recycling unshipped material described in Patent Document 1, at the time of reuse, the amount of impurity present in the unshipped material is estimated and a new single crystal is grown using one or a plurality of pieces of unshipped material.
Here, in prior-art examples in Patent Document 1 and elsewhere, and as shown also in FIG. 8, the resistivities at the top and bottom sides of the reusable ingot are measured, and a single representative value (e.g., an average value) determined from these resistivities is obtained and placed on a label or the like together with the weight of the reusable ingot, which is then stored.
When a single crystal is to be produced using this reusable ingot, the amount of impurity present in the reusable ingot is calculated based on the representative value for the above-described resistivity and the weight value for the reusable ingot, and the reusable ingot is used as the impurity material added to the silicon starting material.
However, in prior-art examples, when the amount of impurity contained in a single reusable ingot is calculated from the representative value, because the reusable ingot as a whole contains on average the impurity in the impurity concentration indicated by the representative value, there is a problem in that the calculated amount of impurity will end up deviating from the amount of impurity actually contained as impurity concentrations having distributions in the single crystal axial and radial directions of the reusable ingot.
That is, in prior-art examples, because the profile (axial direction of single crystal) of the resistivity within the reusable ingot is unclear, the above-mentioned representative value will not necessarily be representative of the actual reusable ingot resistivity, thus making it impossible to calculate the correct amount of impurity and preventing the desired resistivity from being achieved in the single crystal that has been grown.
The present invention was ultimately arrived at in light of the above circumstances. The object of the invention is to provide an impurity amount control system and an impurity amount control method for manufacturing a single crystal which, by accurately estimating the amount of impurity contained within reusable ingots and effectively utilizing the reusable ingots, are able to grow single crystals of a desired resistivity.
Patent Document 1: Japanese Patent Application, First Publication No. 2005-112669.